Section 1: Halftone Printing
Printing typically involves repetitive transfer of ink from a master onto a medium (e.g., paper, cardboard or other physical surface). For color reproduction, four types of inks are typically used, Cyan (C), Magenta (M), Yellow (Y) and Black (K), although of course other types of ink systems can also be used. Many applications, e.g books and other texts, use only black ink. Inks or dyes applied in printing behave as filters that pass only part of the light spectrum hitting the medium and inks. The light incident on the paper is spectrally filtered by the ink layer and reflected back by the medium towards the observer. Each of the primary inks blocks its complementary color, such that in the case of typical inks C passes the green and blue portions of the spectrum and blocks the red portion, M passes the red and blue portions and blocks the green portion and Y passes the red and green portions and blocks the blue portion. Black ink typically blocks the entire spectral range. Thus, upon reflection from the medium surface only part of the spectrum arrives to the eye of the viewer creating the sensation of a certain color. This method of color reproduction is termed subtractive color mixing. The term “subtractive” refers to the creation of color by removing a portion of the spectrum of light transmitted to the eye.
Printing methods are typically binary in nature, namely an ink layer of a uniform, unvarying thickness is either present or absent on the paper surface in certain areas; these areas may be termed “printing dots”. Only two levels of the ink color exist: full ink or bare paper. This limits the amount of colors that can be presented by the inks and their overlaps, and furthermore does not allow for gradations. To obtain “gray levels” for each of the inks, halftone printing may be used. The paper is typically divided by a virtual grid into printing dots. The area of each printing dot is partially or fully covered with ink. The relative area covered by ink is known as the dot area or dot percentage (dot %).
In other print methods, e.g. ink-jet printing, the ink layer thickness is not binary. Nevertheless, the number of gradation levels may be rather limited and halftoning may be used to increase the effective number of levels, and to obtain smoother color transitions.
The blocking characteristics of the ink layer are measured by the ink density, which may be considered the negative logarithm of the transmission in the blocking region of the spectrum of the relevant ink. Higher ink density implies more saturated ink color. If the paper is only partially covered with ink, the apparent density is lower than the density of a solid ink layer, and the color is brighter and less saturated. For example, consider a cyan ink which passes the blue and green components of the spectrum and blocks the red component. The cyan solid ink density is the amount of the red component passing through a full coverage cyan layer. If there are small dots of cyan on paper, which are small enough to be below the eye resolution at typical reading distance, the paper has a pale cyan tint (a “gray level” or gradation of cyan). The apparent density of this tint is lower than that of a solid cyan layer because more of the red component of the white spectrum is received by the eye, since a large amount of the red component of the light is received from the uncovered areas of the paper (assuming a white paper medium). If the density of the tint area is defined in a similar manner as that of the density of the inks, there is a relationship between the tint density, the solid ink density, and the relative area of inked paper to the relative area of non-inked paper.
In practice, the halftone printing method is implemented with an imaginary square grid, dense enough so that the eye typically cannot perceive it from normal viewing distance, dividing the surface of the printing plate. Each elementary cell of this grid is meant to contain a printing dot. The grid reciprocal period (spatial frequency) is called a mesh or raster, and is typically in the range of 60-200 LPI (lines per inch), equivalent to 2.4-7.9 lines per millimeter (for commercial printing); other dimensions can be used. The apparent density is obtained by partial coverage of the area of the printing dot with ink. For the highest density all the area is covered with ink (100% dot area). For zero density the area is not covered by ink (0% dot area).
Each of the inks is layered according to its virtual grid. When examining the printed paper at the usual viewing distance, the impression of color is achieved. However, looking at the printed paper through a magnifying glass resolves a delicate arrangement of dots in the original primary colors, and overlap regions of colors. This can be seen in FIG. 1, showing a full scale sample image and a magnified region of that image, in which the dot structure is revealed. The elementary colors, seen through the magnifying glass, include the four inks CMYK, the three overlaps between two inks each typically producing Red (overlap of M and Y), Green (overlap of C and Y) and Blue (overlap of C and M), and the white color of the typical paper medium. Overlap of CMY produces a black color, and any overlap of C, M or Y with black also produces black (K). In practice, pure black may not be produced by CMY alone for various reasons, but in most cases CMY can be treated as black. Thus, the total effective number of elementary colors is seven, CMY RGB and white/black (white/black is the same color at different brightness levels). Since the CMY RGB and white/black dots are typically not discernible to an unaided eye at the intended difference, the eye integrates (additively) the light reflected back from these colors, creating the sensation of color. Halftone printing may also be used to produce images having only black and white (and shades of gray).
Section 2: Screen and the Moiré Effect
The screening process converts the original image density to printing dot shape and size. In recent years screening has been performed digitally. The apparent densities of the image picture elements (pets) are stored (in terms of dot area) in a digital file containing, for example, CMYK data. Of course, the screening process, and the halftone printing process, may be used with files having other color formats, and with files representing black and white images. Typical tone-depth is 8 bit/separation, representing 256 “gray” levels, although other data formats may be used. A value of zero corresponds to zero coverage of the relevant ink, while a value of 255 corresponds to a full coverage of the relevant ink in the printing dot. The screening process transforms those 256 levels into a bi-level format (one bit data) representing a large number of covered/uncovered spots which are portions of the printing dot. Each printing dot is projected on a matrix of higher resolution than the resolution of the printing dots (determined by the resolution of the plate setter, typically 100 spots/mm), and the spots of this higher density matrix are set on or off according to the required apparent density. Thus each printing dot in the mesh of printing dots is itself represented by a higher density matrix of spots. This is shown in FIG. 2, depicting two sample circular dots of different areas created by exposing certain spots inside an area representing a printing dot. The screened file is a raster bitmap (two-level data) of the image data file after it has been converted according to the procedure described above, where each bit corresponds to the on/off condition of a spot. One screened file may exist for each dye; alternately the data may be combined into one file.
The binary screened file is exposed on the CMYK plates. As a result of the screening process each plate is a periodic grid of printing dots, with a periodicity determined by the mesh. Each printing dot is formed by spots. During printing, plates are impressed on paper, and one or more grids (of the different separations/inks) are laid one on top of another.
If the overlap of more than one grid is not perfect a Moiré pattern may appear. A Moiré pattern is an artificial periodic structure visible to the naked eye, which is created by the interference of two or more periodic patterns, as shown in FIG. 5. The Moiré pattern is a periodic structure of darker and brighter regions (the brighter regions of the Moiré pattern are marked by arrows in FIG. 5) that are clearly visible by the observer. If a Moiré pattern appears in a reproduction (e.g., printed or other material) based on an original image, the viewer at the appropriate distance may see a pattern in the reproduction that does not exist in the original. To minimize these patterns, the multiple grids may be placed at certain angles, for example having 30° between each pair of separations. However, full elimination of the Moiré effect may not be possible, since in one implementation there are a certain number (for example four) separations and only 90° to spread them. As a result, one of the separations, usually yellow, may be placed at 15° with respect to other separation, giving rise to a pattern in certain colors. Other factors may cause Moiré effects. Another source of Moiré patterns can result from interference between the screen of the printing dot and a periodic pattern existing in the image (e.g., a striped shirt or other regular pattern) or other artifacts on the image. Other visible artifacts may appear in a reproduction that are not in the original image; for example blocking, overexposure, or rosettes.
Section 3: Proofing of Printed Material on Electronic Devices
Proofing involves the creation of an accurate apparent match between an original representation of an image (for example, a photograph) and a reproduction of the image (for example, a halftone printed version of the photograph meant for mass distribution). Originals may be, for example, pictorial slides, which are analog in nature; however other types of originals may be proofed. Originals typically have a very high spatial resolution, and therefore can typically be considered continuous. Furthermore, their density gradation varies also continuously. In the age of digital information most of the reproduction process is done digitally. The original is scanned to obtain a file containing the color data (when color images are considered). For example, a typical scanning may result in a file containing RGB values. The file is typically converted to CMYK separations, and plates are created, which are installed on a press for print. To obtain color consistency, proofs are produced and examined in various stages of the process, to assure that each step is color consistent with its previous step. In many cases, a proof representing the halftone print is also prepared, to check for errors in the screening process, or to examine for interference between the screen spatial frequencies and periodic structures in the image, that might result in Moiré or other artifacts.
Proofing usually includes printing a “hard proof” on paper (or other substrate) using the same films that are later used for plate making for printing the final version of the physical material. This paper “hard proof” is sent to the customer and/or designer for approval. If a Moiré pattern or other defect is detected manually (by visual inspection with a naked eye), films are prepared again and the process is repeated. In many cases such a screening proof is not made, and such an error is detected only on the final prints, resulting in a major loss of time and increased cost.
This manual procedure limits the advantages of digital workflow. The need for an accurate digital “soft proof” on an electronic display is clear.
Currently available soft proofing devices enable designers and pre-press personnel to view the works on a computational device such as a personal computer or workstation displays (usually based on Cathode Ray Tubes, or CRTs), while the final product is a printed image on paper. However, these background art devices do not overcome inherent deficiencies for digital print proofing, and in particular cannot accurately replicate, and hence enable the designer and pre-press personnel to detect, such defects and artifacts as Moiré patterns.
As previously described, Moiré patterns of certain periodicity are highly visible and disturb the image perception. They are difficult to predict, since they involve the human perception of low-frequency beats resulting from interference of the screen spatial frequencies (sometimes with the image spatial frequencies). Even if the different beat frequencies might be predicted, their visibility depends on the perception of the eye, which is influenced by the color, brightness, and the surroundings. In practice, Moiré patterns are detected by analog simulation of the print process involving the creation of an image on paper in a similar manner to printing. This “hard” screening-proof method is currently cumbersome, time consuming, inefficient and expensive. A soft proof of screening on an electronic digital display would allow better connectivity to the digital workflow, which is increasingly penetrating the printing industry. However, an effective method for soft screen proofing is not yet available.
Conventional display devices based on cathode ray tubes, plasma screens, or liquid crystal flat panel devices cannot reproduce the details of a print image sufficiently such that print effects are visible to human viewers. One reason is that the size of the pixels is fixed and is relatively large compared to the printing dots and spots of the print process. Furthermore, to present color, each pixel may be combined from a set of red, green and blue sub-pixels. Color is achieved by integration of light coming from neighboring sub-pixels. The display sub-pixel grid cannot typically reproduce the overlapping effect of the grids of the different separations (of CMYK inks), since color presentation in these devices is inconsistent with halftone presentation.
Therefore, there is an unmet need for a device, system and a method for accurate electronic display of an image to be printed in order to provide soft proofing of an image before being printed, particularly for detection of potential artifacts.